1. Field of the Invention
The present invention relates to a printing apparatus having a line-type ink jet head and a method of printing images by a line-type ink jet head.
2. Description of the Background Art
Japanese Patent Published Application No. Hei 8-132645 discloses a printing apparatus provided with a line-type ink jet head. The line-type ink jet head has a length covering the same print width as that of print sheet, and fixedly mounted on the housing of the printing apparatus. The ink jet head discharges ink to a print sheet which is transported in the direction (vertical scanning direction) perpendicular to the line direction (main scanning direction) for forming a print image on the print sheet.
FIG. 1A is a conventional line-type ink jet head comprising a plurality of head blocks 230 (six blocks in this example) arranged in the line direction. FIG. 1B is a view for explaining the configuration of the block 230 of the conventional line-type ink jet head shown in FIG. 1A, and the dot pattern of ink ejected from the nozzle units of the line-type ink jet head. Each block 230 is composed of nozzle units 240a and 240b joined together each of which is elongated in the line direction and provided with a number of nozzles as illustrated in FIG. 1B. These nozzles are divided into groups each of which comprises three nozzles obliquely arranged with respect to the line direction for the purpose of increasing the resolution.
As shown in FIG. 1B, the nozzle unit 240a and the nozzle unit 240b are joined with a displacement p1 in the line direction. The displacement p1 is equal to ½ of the pitch dp of the nozzle such that each nozzle of the nozzle unit 240b is located between adjacent nozzles of the nozzle unit 240a in the line direction. By this configuration, ink dots can be placed by the combination of the nozzles of the nozzle unit 240a and nozzle unit 240b at even intervals corresponding to dp/2 in order to form a uniform image with even densities.
However, since the ink jet head 220 is manufactured as an industrial good, there may be some variations among products. Because of this, there may be a block in which the distance p2 between the nozzle unit 240a and the nozzle unit 240b is smaller (as illustrated in FIG. 2A) or larger than ½ of the pitch dp of the nozzle. The nozzle pitch periodically varies in such a block 230 in the line direction. Namely, adjacent nozzles of the nozzle unit 240a and nozzle unit 240b are paired with a smaller distance therebetween, such that the distance between adjacent pairs increases.
As illustrated in FIG. 2B, the block 230 may be obliquely arranged with respect to the line direction. The nozzle pitch periodically varies in such a block 230 also in such a block 230. Namely, also in this case, adjacent nozzles of the nozzle unit 240a and nozzle unit 240b are paired with a smaller distance therebetween, such that the distance between adjacent pairs increases.
It is possible to form the block 230 by making use of a single nozzle unit 240 having two series of nozzle groups as illustrated in FIG. 2C in place of the pair of nozzle units 240a and 240b joined together. In such a case, while the uneven dot arrangement due to the misalignment between the two nozzle units 240a and 240b joined together can be removed from the block 230 shown in FIG. 2A, the uneven dot arrangement due to oblique arrangement of the block 230 cannot be avoided as illustrated in FIG. 2C.
Incidentally, the ink jet printing apparatus controls the gradation level by changing the number of droplets to be ejected for each pixel (dot) from each nozzle. For example, a lighter pixel of an image is represented by a smaller number of droplets while a darker pixel of an image is represented by a larger number of droplets. Also, the amount of ink discharged from the nozzle corresponding to one droplet is in proportion to the voltage pulse applied to the piezoelectric element of the nozzle, and can be adjusted independently for each block.
FIG. 3A is a schematic diagram for showing the differences between the print image formed by an imperfect block 230 having nozzles misaligned with periodic pitch displacements from correct positions and the print image formed by a normal block 230 having nozzles arranged in their correct positions. The upper image is a light solid image portion printed by ejecting one droplet for each pixel, and the lower image is a dark solid image portion printed by ejecting seven droplets for each pixel. The left half of each image is printed by the normal block 230, and the right half of each image is printed by the imperfect block 230.
As shown in the same figure, the light solid image portion printed by ejecting one droplet for each pixel is less affected by the uneven dot arrangement, and there is little difference in density between the print image formed by the imperfect block 230 having nozzles misaligned with periodic pitch displacements and the print image printed by the normal block 230 having nozzles arranged in their correct positions. On the other hand, the dark solid image portion printed by ejecting seven droplets for each pixel is substantially affected by the uneven dot arrangement. The density is lowered by the uneven dot arrangement as compared with that of the dark solid image printed by the normal block 230.
In order to adjust the differential density, it is thought to increase the voltage of the pulses applied to the piezoelectric elements of the block 230 having nozzles misaligned with periodic pitch displacements. However, if the voltage of the pulses is increased, the densities of images are increased whenever the images are printed by the block 230 with the uneven dot arrangement, and thereby as illustrated in FIG. 3B there is a differential density in the light solid image portion between the print image formed by the imperfect block 230 and the print image printed by the normal block 230 while the differential density in the dark solid image can be eliminated.